1. Field of the Invention
The present invention relates to a power generating system including fuel cell power generating units and, more specifically, to a power generating system including fuel cell power generating units using hydrogen gas obtained by reforming natural gas or the like and supplied thereto through piping, capable of smoothly responding to the variation of power demand.
2. Description of the Prior Art
It is generally known to generate electric power by fuel cells using hydrogen gas obtained by reforming a gas, such as natural gas or naphtha. A power generating system comprising the combination of a reforming unit and a fuel cell unit is a convenient means of electric power generation in remote regions and isolated islands.
The fuel cell is made to generate electric power simply by supplying hydrogen and oxygen (air). The fuel cell converts the chemical energy of a fuel directly into electric energy in a continuous process at an efficiency higher than that of other power generating systems, such as a so-called combined cycle power generating system employing, in combination, a gas turbine and a steam turbine, is capable of readily responding to the variation of power demand.
On the other hand, the reformer which converts natural gas or the like into gas containing hydrogen gas as a principal component by using a reforming medium, such as oxygen gas or steam, needs a comparatively long time for start-up, namely, the time required for the reformer to reach a steady state, and a comparatively long time for shut-down, namely, the time required for the reformer to change from a steady state to a stop, and, hence, the reformer is unable to operate at a high efficiency unless the reformer is kept continuously in the steady state.
Power demand is variably daily, weekly, monthly as well as between seasons. The daily power demand, for example, has a peak power demand in the afternoon, a minimum power demand between midnight and morning, and an average power demand between the peak power demand and the minimum power demand. Since it is difficult or disadvantageous to store electric energy, power plants must have a capacity corresponding to the peak power demand, which necessarily entails loss in power transmission.
Accordingly, even a fuel cell power generating system including a fuel cell unit capable of generating electric power at an appropriate rate in response to the variation of power demand is not economically effective when the fuel cell power generating system employs a reformer, which ordinarily includes a purifier or purification unit, having a capacity corresponding to the peak power demand to cope with the variation of power demand, because such a fuel cell power generating system requires a large investment and is unable to operate efficiently.
The greater is the capacity of the reformer or the longer the period of operation of the reformer in the steady state, the higher is the efficiency of the reformer. Additionally, techniques relating to the reformer have almost completely been established and have been proved in chemical plants. However, it is very difficult to quickly start up or shut down a reformer having a smaller capacity or to regulate the hydrogen-generating rate of the reformer according to the condition of the individual fuel cell power generating units. Furthermore, the reformer, as a chemical plant, needs additional space for installation and requires skilled operators, which an economically disadvantageous.